Answer
$10.\%(\text{m/v})$ acetic acid solution.
Work Step by Step
$C_{1}=50.0\%(\text{m/v})$
$V_{1}=5.0\,mL$
$V_{2}=25\,mL$
Rearranging the dilute expression $C_{1}V_{1}=C_{2}V_{2}$, we get
$C_{2}=\frac{C_{1}V_{1}}{V_{2}}=\frac{50.0\%\times5.0\,mL}{25\,mL}=10.\%(\text{m/v})$